Buggy code:
public final void beforeRender ( ) { } public final boolean continueToOriginalDestination ( ) { } public final void debug ( final String message ) { } public final void detach ( ) { } public final void detachBehaviors ( ) { } public void detachModels ( ) { } public final void error ( final Serializable message ) { } public final void fatal ( final String message ) { } public final MarkupContainer findParent ( final Class c ) { } public final MarkupContainer findParentWithAssociatedMarkup ( ) { } public final Application getApplication ( ) { } public final List getBehaviors ( ) { } public final String getClassRelativePath ( ) { } public final IComponentBorder getComponentBorder ( ) { } public final IConverter getConverter ( ) { } public IConverter getConverter ( Class type ) { } public final boolean getEscapeModelStrings ( ) { } public final FeedbackMessage getFeedbackMessage ( ) { } public String getId ( ) { } public final IModel getInnermostModel ( ) { } public Locale getLocale ( ) { } public final Localizer getLocalizer ( ) { } public final ValueMap getMarkupAttributes ( ) { } public String getMarkupId ( ) { } public final Serializable getMetaData ( final MetaDataKey key ) { } public final IModel getModel ( ) { } public final Object getModelObject ( ) { } public final String getModelObjectAsString ( ) { } public final String getModelObjectAsString ( final Object modelObject ) { } public final boolean getOutputMarkupId ( ) { } public final Page getPage ( ) { } public final IPageFactory getPageFactory ( ) { } public final String getPageRelativePath ( ) { } public final MarkupContainer getParent ( ) { } public final String getPath ( ) { } public final boolean getRenderBodyOnly ( ) { } public final Request getRequest ( ) { } public final RequestCycle getRequestCycle ( ) { } public final Response getResponse ( ) { } public Session getSession ( ) { } public long getSizeInBytes ( ) { } public final String getString ( final String key ) { } public final String getString ( final String key , final IModel model ) { } public final String getString ( final String key , final IModel model , final String defaultValue ) { } public final String getStyle ( ) { } public String getVariation ( ) { } public final boolean hasBeenRendered ( ) { } public final boolean hasErrorMessage ( ) { } public final boolean hasFeedbackMessage ( ) { } public final void info ( final String message ) { } public final void internalAttach ( ) { } public final void internalDetach ( ) { } public final boolean isActionAuthorized ( Action action ) { } public final boolean isAncestorOf ( final Component component ) { } public final boolean isEnableAllowed ( ) { } public boolean isEnabled ( ) { } public final boolean isRenderAllowed ( ) { } public final boolean isStateless ( ) { } public boolean isVersioned ( ) { } public boolean isVisible ( ) { } public final boolean isVisibleInHierarchy ( ) { } public final void markRendering ( ) { } public final void modelChanged ( ) { } public final void modelChanging ( ) { } public final Page newPage ( final Class c ) { } public final Page newPage ( final Class c , final PageParameters parameters ) { } public void prepareForRender ( ) { } public final void redirectToInterceptPage ( final Page page ) { } public final void remove ( ) { } public Component remove ( final IBehavior behavior ) { if ( behavior == null ) { throw new IllegalArgumentException ( "Argument<seq2seq4repair_space>`behavior`<seq2seq4repair_space>cannot<seq2seq4repair_space>be<seq2seq4repair_space>null" ) ; } <START_BUG> if ( ( ( behaviors ) == null ) || ( ! ( behaviors . contains ( behavior ) ) ) ) { <END_BUG> throw new IllegalStateException ( ( "Tried<seq2seq4repair_space>to<seq2seq4repair_space>remove<seq2seq4repair_space>a<seq2seq4repair_space>behavior<seq2seq4repair_space>that<seq2seq4repair_space>was<seq2seq4repair_space>not<seq2seq4repair_space>added<seq2seq4repair_space>to<seq2seq4repair_space>the<seq2seq4repair_space>component.<seq2seq4repair_space>Behavior:<seq2seq4repair_space>" + ( behavior . toString ( ) ) ) ) ; } if ( ! ( behavior . isTemporary ( ) ) ) { addStateChange ( new Component . RemovedBehaviorChange ( behavior ) ) ; } behaviors . remove ( behavior ) ; if ( ( behaviors . size ( ) ) == 0 ) { behaviors = null ; } return this ; } public final void render ( ) { } public final void render ( final MarkupStream markupStream ) { } public final void renderComponent ( ) { } public final void renderComponent ( final MarkupStream markupStream ) { } public final void rendered ( ) { } public void renderHead ( final HtmlHeaderContainer container ) { } public void replaceWith ( Component replacement ) { } public final boolean sameInnermostModel ( final Component component ) { } public final boolean sameInnermostModel ( final IModel model ) { } public final Component setComponentBorder ( final IComponentBorder border ) { } public final Component setEnabled ( final boolean enabled ) { } public final Component setEscapeModelStrings ( final boolean escapeMarkup ) { } public void setMarkupId ( String markupId ) { } public final void setMetaData ( final MetaDataKey key , final Serializable object ) { } public Component setModel ( final IModel model ) { } public final Component setModelObject ( final Object object ) { } public final Component setOutputMarkupId ( final boolean output ) { } public final Component setOutputMarkupPlaceholderTag ( final boolean outputTag ) { } public final void setRedirect ( final boolean redirect ) { } public final Component setRenderBodyOnly ( final boolean renderTag ) { } public final void setResponsePage ( final Class cls ) { } public final void setResponsePage ( final Class cls , PageParameters parameters ) { } public final void setResponsePage ( final Page page ) { } public Component setVersioned ( boolean versioned ) { } public final Component setVisible ( final boolean visible ) { } public String toString ( ) { }

Fix:
if ( ! ( behaviors . contains ( behavior ) ) ) {